1. Field of the Invention
The present invention generally relates to applications of action of stimulated Brillouin scattering to optical systems and, more particularly, relates to optical signal generators, optical communication systems, and methods for searching for a Fresnel reflection point in an optical transmission line arranged by making use of the phenomenon of stimulated Brillouin scattering.
The stimulated Brillouin scattering is a nonlinear optical phenomenon and it has not been positively utilized in such fields as optical communication. Rather, there have been made proposals to avert the adverse effect of the stimulated Brillouin scattering as a factor producing deteriorated transmission characteristics. Namely, it may safely be said that it has been a technological subject how to suppress the generation of the stimulated Brillouin scattering. The phenomenon of stimulated Brillouin scattering has such peculiar characteristics that: (a) the threshold value for it is extremely lower (by one to two digits) than that for other nonlinear optical effect (1 mW or so in the case of a single mode fiber whose loss is 0.2 dB/km or so); (b) since it is produced through interaction between incident light and acoustic phonon, the Stokes' shift is small (for example, approximately 11 GHz at 1.55 .mu.m band in the case of a single mode silica fiber); and (c) only backward scattered light is mainly produced therein. Accordingly, it is considered that a variety of applications of it to optical systems would be achieved if such characteristics could be advantageously utilized.
2. Description of the Related Art
There have been few proposals for optical systems arranged by positively utilizing the effects of stimulated Brillouin scattering except that an application of it to an optical filter making use of the narrow band characteristic of the stimulated Brillouin scattering and an application of stimulated Brillouin scattered light and exciting light to wavelength division multiplexing have only been contemplated. The fact that there are made only a few applications of the stimulated Brillouin scattering is considered to be due to the narrowness of the band for the stimulated Brillouin scattering. Since the stimulated Brillouin scattering depends on interaction of incident light with an acoustic phonon, the band is determined by the relaxation time of the acoustic phonon. Accordingly, the band at normal temperature is on the order of 10 to 100 MHz. Because of this narrowness in bandwidth, substantially no application of it to broadband transmission such as optical communication has been contemplated.